Slot 1 cards plug into the slot 1 and is all thats needed to play commercial DS roms
Slot 2 cards plug into the slot 2, and needs either a FlashMe firmware replacement or a passcard to operate, however has GBA and DS support. Slot 1 is becoming increasingly popular though.

For newer cards, all you need is a microSD and a suitable reader, and the slot1/2 card itself. If anything else is necessary - it will be included.

Does the R4 card need a passcard in order to play commercial NDS games? can it run NDS homebrew? is there a way to make 1-slot to play gba roms? I don't wanna spend more money just to buy 1-slot & 2-slot card. Would these cards work on a Australian NDS Lite?

Does the R4 card need a passcard in order to play commercial NDS games?No.

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can it run NDS homebrew?Yes, if the homebrew uses DLDI, or does not need filesystem access. Old fatlib homebrew will not be able to access files on the microSD card. Nearly all new homebrew works.

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is there a way to make 1-slot to play gba roms? I don't wanna spend more money just to buy 1-slot & 2-slot card.There is no way, it is completely impossible. Get the EZ-V and the 3 in 1 cart if you want good DS and GBA support as cheap as possible.